The Sublime Attitudes: A Study Guide on the Brahmaviharasby Ṭhānissaro Bhikkhu.

The sublime attitudes (brahmaviharas) are the Buddha’s primary heart teachings—the ones that connect most directly with our desire for true happiness. They’re the qualities of heart that motivated the Buddha to find awakening and then to teach the path of awakening to others. At the same time, they function as part of the path itself.

...And what is lack of food for the arising of unarisen sloth & drowsiness, or for the growth & increase of sloth & drowsiness once it has arisen? There is the potential for effort, the potential for exertion, the potential for striving. To foster appropriate attention to them: This is lack